1. Field of the Invention
The present invention relates to a primary radiator provided in a satellite receiving reflective antenna or the like and, in particular, to a primary radiator using a dielectric feeder.
2. Description of the Related Art
FIG. 6 is a sectional view of a conventional primary radiator using a dielectric feeder. This primary radiator comprises a wave guide 1 one end of which is open and the other end of which is formed as a closed surface e 1. Inside the wave guide 1, a first probe 3 and a second probe 4 are installed so as to be orthogonal to each other. The distances between the probes 3 and 4 and between the probe 3 and the closed surface 1a are approximately xc2xc of the guide wavelength. The dielectric feeder 2 is formed of a dielectric material, such as polyethylene. A holding portion 2a is formed in the middle of the dielectric feeder 2, and a radiation portion 2b and a conversion portion 2c are formed on either side of it. The outer diameter of the holding portion 2a is substantially the same as the inner diameter of the wave guide 1. By forcing the holding portion 2a into the open end portion of the wave guide 1, the dielectric feeder 2 is secured inside the wave guide 1. Both the radiation portion 2b and the conversion portion 2c have a conical configuration, and the radiation portion 2b protrudes externally from the open end of the wave guide 1, the conversion portion 2c extending into the interior of the wave guide 1.
The primary radiator, constructed as described above, is installed at the focal position of the reflecting mirror of a satellite receiving reflective antenna. A radio wave transmitted from the satellite converges at the dielectric feeder 2 and undergoes impedance matching before entering the wave guide 1. And, of the linearly polarized wave input to the wave guide 1, consisting of horizontally polarized wave and vertically polarized wave, the horizontally polarized wave is received by the first probe 3, and the vertically polarized wave is received by the second probe 4. The reception signal is frequency-converted into an IF frequency signal by a converter circuit (not shown) before being output.
Compared with a conical horn type primary radiator having a wave guide whose open end portion is flared, the conventional primary radiator using a dielectric feeder, constructed as described above, is advantageous in that a reduction in radial dimension can be achieved. However, due to the radiation portion 2b and the conversion portion 2c formed at either end of the dielectric feeder 2 and having a conical configuration, the total length of the dielectric feeder 2 is rather large. In particular, the conversion portion 2c extending into the wave guide 1 must be formed as a long cone in order to secure a satisfactory impedance matching with the wave guide 1. Further, the holding portion 2a forced into the wave guide 1 must be long enough to stabilize the dielectric feeder 2 inside wave guide 1, with the result that a reduction in the size of the primary radiator is prevented.
In accordance with the present invention, there is formed at the end surface of the holding portion secured to the inner surface of the wave guide a recess extending in the axial direction of the wave guide or a protrusion having a height of approximately xc2xc of the wavelength of the radio wave. In this construction, the recess or the protrusion formed at the end surface of the holding portion functions as an impedance conversion portion, so that, in spite of the fact that a sufficient length is secured for the holding portion to stabilize the dielectric feeder, it is possible to reduce the total length of the dielectric feeder, making it possible to achieve a reduction in the size of the primary radiator.
In accordance with the present invention, there is provided a primary radiator comprising a wave guide having at one end an opening for introducing a radio wave, and a dielectric feeder held at the open end of the wave guide, wherein the dielectric feeder comprises a radiation portion protruding from the open end of the wave guide and a holding portion secured to the inner surface of the wave guide, a recess extending in the axial direction of the wave guide being formed at the end surface of the holding portion.
In this construction, the impedance matching of the wave guide and the dielectric feeder is effected in the recess extending inwardly from the end surface of the holding portion, so that it is possible to secure a sufficient length for the holding portion to stabilize the dielectric feeder, and reduce the total length of the dielectric feeder to achieve a reduction in the size of the primary feeder.
In the above construction, the recess may have a conical or a pyramid-like configuration tapering off toward the interior of the dielectric feeder. To reduce the depth of the recess, however, it is desirable to form it as a cylindrical hole having a depth of approximately xc2xc of the wavelength of radio wave, or a stepped hole consisting of a plurality of continuously formed cylindrical holes having different diameters, the depth of each cylindrical hole approximately xc2xc of the wavelength of radio wave. In this case, in each cylindrical hole, the phase of the radio wave reflected at the bottom surface and the open end of the cylindrical hole is reversed to be canceled, so that it is possible to substantially reduce the reflection component of the radio wave, and the impedance matching with the wave guide is effected satisfactorily.
There is no particular restriction to the number of recesses. However, when forming a single recess at the end surface of the holding portion, it is desirable for the recess to be matched with the position of the axial center of the wave guide. On the other hand, when forming a plurality of recesses at the end surface of he holding portion, it is desirable to provide the recesses in an annular arrangement around the axis of the wave guide, or provide the recesses symmetrically with respect to the axis of the wave guide.
In the above construction, when there are formed at the end surface of the radiation portion a plurality of annular grooves having a depth corresponding of xc2xc of the wavelength of radio wave, it is possible to reduce the length of the radiation portion and further reduce the size of the primary radiator.
In accordance with the present invention, there is further provided a primary radiator comprising a wave guide having at one end an opening for introducing radio wave, and a dielectric feeder held at the open end of the wave guide, wherein the dielectric feeder includes a radiation portion protruding from the open end of the wave guide and a holding portion forced into the interior of the wave guide, a protrusion having a height of approximately xc2xc of radio wave being formed at the end surface of the holding portion.
In this construction, the phase of the radio wave reflected at the protruding surface of the protrusion and the bottom surface is reversed to be canceled, so that the reflection component of the radio wave is substantially reduced and a satisfactory impedance matching with the wave guide is ensured, whereby it is possible to restrain the protruding amount of the protrusion functioning as the impedance conversion portion to reduce the total length of the dielectric feeder, thereby achieving a reduction in the size of the primary radiator.
In the above construction, there is no particular restriction to the number of protrusions. However, when forming a single protrusion at the end surface of the holding portion, it is desirable to match this protrusion with the position of the axis of this wave guide. On the other hand, when forming a plurality of protrusions at the end surface of the holding portion, a stepped protrusion consisting of a plurality of continuously formed cylindrical portions having different diameters is formed, the height of each cylindrical portion corresponding to approximately xc2xc of the wavelength of radio wave.